Conventionally, a silicon (SiO2) film has been used as the gate oxide film in semiconductor devices. Nevertheless, since the recent trend of semiconductor devices is significantly inclined toward the realization of thinner films and the miniaturization thereof, particularly, if the gate oxide film is made thinner, problems such as insulation resistance, boron penetration, gate leak, gate depletion and the like will arise.
Thus, conventional silicon will be no longer efficient, and a material having a higher dielectric constant than silicon must be employed. As such a material, the use of ZrO2 and HfO2 having a high dielectric constant of roughly 20 and which are difficult to mix with Si may be considered.
Nonetheless, since ZrO2 and HfO2 have a downside in that they easily pass oxidants through and form a interfacial phase with a small dielectric constant during the deposition or in the annealing process thereafter, the use of ZrSi2 and HfSi2 instead of ZrO2 and HfO2 may also be considered.
When using these materials as the gate oxide film or silicide film, the Zr or Hf target or the silicide targets thereof may be formed by performing sputtering under an atmosphere of inert gas such as argon or under a reactive gas atmosphere. Here, Zr and Hf will be the main raw material in either case.
Meanwhile, in order for a material used in the semiconductor device to guarantee the operational performance as a reliable semiconductor, it is essential that impurities harmful to semiconductor devices within the aforementioned materials formed after sputtering be reduced as much as possible.
In other words,
(1) alkali metal elements such as Na, K;
(2) radioactive elements such as U, Th;
(3) transition metals or heavy metals or high melting point metal elements such as Fe, Ni, Co, Cr, Cr; and
(4) gas component elements such as C, O, N, H must be reduced as much as possible in order to achieve a purity of 4N; that is, 99.99% (parts by weight). Moreover, the %, ppm, ppb used in the present specification all refer to weight %, weight ppm, weight ppb.
Impurities existing in the foregoing semiconductor devices; for instance, alkali metals such as Na, K, transfer easily within the gate insulation film and cause the deterioration of the MOS-LSI boundary characteristics, and radioactive elements such as U, Th cause software errors in devices due to the a ray emitted from such elements. Further, transitional metals or heavy metals or high melting point metal elements such as Fe, Ni, Co, Cr, Cr contained as impurities are known to cause problems in the interface bonding.
Moreover, gas components such as C, O, N, H. which are often disregarded, cause the generation of particles during sputtering, and may be considered as being unfavorable.
A generally available raw material zirconium sponge having a purity level of 3N contains a large quantity of impurities as shown in Table 1; for example, Co: 10 ppm, Cr: 50 ppm, Cu: 10 ppm, Fe: 50 ppm, Mn: 25 ppm, Nb: 50 ppm, Ni: 35 ppm, Ta: 50 ppm, C: 2000 ppm, O: 5000 ppm, N: 200 ppm, etc.
Moreover, a generally available raw material hafnium sponge having a purity level of 2N contains a large quantity of impurities as shown in Table 3, for instance, Cd: 30 ppm, Co: 10 ppm, Cr: 150 ppm, Cu: 50 ppm, Fe: 300 ppm, Mn: 25 ppm, Nb: 30 ppm Ni: 75 ppm, Ta: 100 ppm, O: 500 ppm, N: 60 ppm, etc.
These impurities all hinder the operational performance of semiconductors, and it is necessary to effectively eliminate these impurities harmful to such semiconductor devices.
Nevertheless, there have hardly been any conventional achievements of using zirconium or hafnium as the gate oxide film in a semiconductor device, and, since the refining technology for eliminating such impurities is peculiar and costly, it could be said that no consideration was given to such use of zirconium or hafnium and the use of these materials has been abandoned heretofore.
In addition, although zirconium or hafnium is generally highly purified with electron beam melting, when manufacturing a sputtering target with the aforementioned highly purified zirconium or hafnium as the raw material, it is necessary to manufacture powder of such highly purified zirconium or hafnium.
Thus, an additional process of casting the high-purity zirconium or hafnium obtained with the foregoing electron beam melting into an ingot, and preparing a powder thereafter becomes necessary. The disintegration from an ingot is usually performed with crushing, and, since high-purity zirconium or hafnium powder may ignite and explode, disintegration from an ingot is dangerous. Due to the foregoing reasons, the process of disintegration from an ingot requires sufficient management, and therefore resulted in a problem of increased costs.